Full text
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| Author(s): |
Bruna Passarelli Ricardi
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | São Paulo. |
| Institution: | Universidade de São Paulo (USP). Instituto de Geociências (IG/BT) |
| Defense date: | 2010-04-26 |
| Examining board members: |
Silvio Roberto Farias Vlach;
José Affonso Brod;
Gaston Eduardo Enrich Rojas
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| Advisor: | Silvio Roberto Farias Vlach |
| Abstract | |
The miaskitic to intermediate nepheline syenites of Poços de Caldas Alkaline Massif, especially the Pedreira type, have intriguing felsic and mafic-ultramafic microgranular enclaves. The Pedreiras nepheline syenite type (NeS) can be divided into two textural facies: one that is medium-coarse to coarse grained (NeS-c) and the other that varies between medium-fine to fine grained (NeS-f). It is common among the NeS-c microgranular felsic enclaves (MFE) with phonolitic composition. These enclaves may develop smaller ones maficultramafic, generating double enclaves. The NeS-f usually presents mafic-ultramafic enclaves (MME), sometimes showing disrupted features in plastic stage and sometimes angular and linear edges. Phonolitic dykes cut the NeS-c. The rocks studied in this work are formed by nepheline, alkali-feldspar and clinopyroxene. The accessory phase is characterized by titanite, magnetite and biotite-phlogopite. In exception to the NeS-c, apatite also occurs as an accessory mineral. In the NeS-c, NeS-f and MFE, the clinopyroxene presents two textural phases: a green prismatic one (aegirine-augite), which also may have a pinkish/colourless core (diopsyde, which is common in the NeS-f and MFE); the other one is fibrous, also green, however with strongly pleocroism (aegirine). Essencially, the MME are formed by diopsyde, with M(medium)~80. When porphyritic, the enclave has macrocrystals of pinkish/colourless diopsyde (with mg#~0,9) in a green diopsyde (mg#~0,8) matrix, both prismatic. In the NeS, the magnetite varies little: with low levels of Ti, differently from the enclaves magnetite. The chemical signature of the nepheline in the MME equals to the one present in the NeS-f and has more Fe3+ and less K when compared to the NeS-c. The alkaline feldspar in the MFE has more potassium in its structure and presents a higher compositional variation in the MME (Ab10-33Or72-80). It is also noticeable that biotite is a component only to the MFE, while the phlogopite occurs in the MME. The rare earth elements (REE) pattern in the pinkish/colourless clinopyroxene of the NeS-f is similar to the MME. Both aegirine-augite and aegirine present an enrichment regarding the heavy REE. The MME are ultrabasic, classified as tephrite/basanite (Le Bas et al., 1986) or nephelinite/ankaratrites (De La Roche et al., 1980), while the felsic rocks are intermediate, corresponding to phonolite and nepheline syenite, depending on the grain size. The dykes and the NeS-f are peralkaline, while the MFE, NeS-c and the MME are peralkaline/metaluminous. The ultrabasic/ultrapotassic rocks, outcropping in the northwestern portion of the PCAM (Ulbrich et al., 2002), have REE patterns wich indicate that these rocks may be genetically related to mafic-ultramafic enclaves. Generally, the structural, textural and chemical signatures of the rocks studied in this work confirm the hypothesis of the coexistence of at least two different magmas: a syenitic undersaturated felsic one and a ultramafic/ultrabasic one, and their interaction resulted in the MME and, above all, the NeS-f. While in a late stage of magma cristalization, but still in the plastic state, the dike would be placed, with partial absorption of the rock by the nepheline syenite, resulting in the EMF. (AU) | |